1. Field of the Invention
The present invention relates to a distributed amplifier. For example, the distributed amplifier of the present invention is applied to broadband voltage amplifiers used in optical communications systems or the like.
2. Description of Related Art
Conventionally, distributed amplifiers have been known as amplifiers that amplify broadband signals. For example, the xe2x80x9cA 69 GHz broadband Distributed Amplifierxe2x80x9d described on page 53 of Preprints of the 2001 Electronics Society Conference of the Electronic Information and Communications Society [Denshi Joho Tsushin Gakkai] (Ogawa et al.) is known as a distributed amplifier.
FIG. 7 is a circuit diagram which shows the construction of such a distributed amplifier. The distributed amplifier 700 comprises source-grounded field effect transistors 701-1 through 701-8 and gate-grounded field effect transistors 702-1 through 702-8. The drains of the source-grounded transistors 701-1 through 701-8 are respectively connected to the sources of the corresponding gate-grounded transistors 702-1 through 702-8. Amplifying circuits that combine source-grounded amplifying transistors and gate-grounded amplifying transistors are called xe2x80x9ccascode amplifying circuitsxe2x80x9d. In the example shown in FIG. 7, eight cascode amplifying circuits are provided. Specifically, this distributed amplifier 700 has an eight-section construction.
The sources of the source-grounded transistors 701-1 through 701-8 are connected in common to a ground line. Furthermore, the gates of the gate-grounded transistors 702-1 through 702-8 are connected in common to a VC power supply.
The gates of the gate-grounded transistors 701-1 through 701-8 are connected to a signal input terminal 711. A signal IN is input from the signal input terminal 711.
The drains of the gate-grounded transistors 702-1 through 702-8 are connected to a signal output terminal 712. A signal OUT is output from the signal output terminal 712. A power supply potential VDD is applied to this signal output terminal 712 by means of an external bias circuit not shown in the figures.
Coplanar transmission lines 703-1 through 703-8, 704-1 through 704-8, 705-1 through 705-8 and 706-1 through 706-8 are used as transmission paths that are connected between the source-grounded transistors 701-1 through 701-8 and the gate-grounded transistors 702-1 through 702-8.
The transmission line consisting of the coplanar transmission lines 703-1 through 703-8 is connected to a ground line via a terminating resistance 707 and a capacitor 708. A bias input terminal 713 is connected between the terminating resistance 707 and capacitor 708. The bias input terminal 713 is used for the external connection of another capacitor in cases where the capacitance of the capacitor 708 in insufficiently large; furthermore, this bias input terminal 713 is also used to supply the gate bias TMI of the source-grounded transistors 701-1 through 701-8. Here, when the gate bias TMI is supplied, an external circuit (not shown in the figures) must be connected to the bias input terminal 713 in order to cut the direct-current component.
The transmission line consisting of the coplanar transmission lines 706-1 through 706-8 is connected to a ground line via a terminating resistance 709 and a capacitor 710. A terminal 714 is connected between the terminating resistance 709 and the capacitor 710. This terminal is used for the external connection of another capacitor in cases where the capacitance of the capacitor 710 in insufficiently large. Here, a terminating resistance (not shown in the figures) is externally connected to the signal output terminal 712. Specifically, in this distributed amplifier 700, two output side terminating resistances are used. These output side terminating resistances are connected in parallel as seen from the side of the gate-grounded transistors 702-1 through 702-8.
The circuit thus constructed can be caused to function as a broadband amplifier by appropriately setting the potentials VDD, VC and TMI. The voltage gain Gv of this distributed amplifier is given by Equation (1) below. In Equation (1), n is the section number, and gm is the mutual conductance per section. Furthermore, RL/2 is the synthesized value of the two output side terminating resistances.
Gv=nxc3x97gmxc3x97RL/2xe2x80x83xe2x80x83(1)
Ordinarily, the voltage gain Gv of the distributed amplifier is set by varying the potential TMI, i.e., the gate bias of the source-grounded transistors 701-1 through 701-8. Varying the potential TMI causes that voltages across the gates and sources of the respective source-grounded transistors 701-1 through 701-8 to vary; as a result, the mutual conductance gm varies, so that the voltage gain Gv can be varied. In cases where the potential TMI is used, the voltage gain Gv can be continuously varied from zero to the maximum value Gvmax.
However, the distributed amplifier shown in FIG. 7 suffers from the following drawback: specifically, when the potential TMI is varied in order to reduce the voltage gain Gv, there is also a variation in the output signal waveform. Such waveform variation is usually a problem for the device that receives such an output signal. The effect of this drawback is especially conspicuous in the case of distributed amplifiers used to amplify base-band digital signals used in optical communications devices and the like.
FIGS. 8A-8D show simulated results for the input waveform and output waveform of the distributed amplifier 700. In this simulation, GaAs Pseudomorphic HEMTs (high electron mobility transistors) with a gate length of 0.1 m and a gate width of 40 m were used as the respective transistors 701-1 through 701-8 and 702-1 through 702-8.
FIG. 8A shows the waveform of the input signal IN (see FIG. 7). The input signal IN is a 40 Gbps seven-stage quasi-random signal with an amplitude of 0.5 volts (i.e., 0.5 Vpp). A waveform of the type shown in FIG. 8A is called an eye pattern. When a waveform is evaluated using an eye pattern, the position of the cross point between the rising portion and falling portion of the signal is an important parameter. Specifically, it may be said that the deterioration of the signal waveform becomes more severe as the positional deviation of the cross point increases. In most cases, as is shown in FIG. 8A the cross point of the input signal IN is set so that this cross point is positioned substantial center between the high level and the low level.
FIG. 8B shows the waveform of the output signal OUT that was obtained when the potential TMI was set at zero volts. The voltage gain Gv in this case was 3.4. As is seen from FIG. 8B, the position of the cross point of the output signal OUT, like that of the cross point of the input signal IN (see FIG. 8A) is more or less the center between the high level and low level.
FIG. 8C shows the waveform of the output signal OUT that was obtained when the potential TMI was set at xe2x88x920.25 volts. The voltage gain Gv in this case was 3.2. As is seen from FIG. 8C, the position of the cross point of the output signal OUT is slightly higher than the positions of the cross point in the waveforms shown in FIGS. 8(A) and 8(B).
FIG. 8D shows the waveform of the output signal OUT that was obtained when the potential TMI was set at xe2x88x920.50 volts. The voltage gain Gv in this case was 2.2. As is seen from FIG. 8D, the position of the cross point of the output signal OUT is more higher than the position of the cross point in the waveform shown in FIG. 8C.
The reasons for such deviation of the cross point will be described below with reference to FIG. 9. FIG. 9 is a graph which shows the relationship between the gate-source voltage Vgs and the mutual conductance gm in the source-grounded transistors 701-1 through 701-8. In FIG. 9, the operating points b, c and d correspond respectively to FIGS. 8B, 8C and 8D.
As is seen from the abovementioned Equation (1), the mutual conductance gm is proportional to the voltage gain Gv; accordingly, if the mutual conductance gm increases, the voltage gain Gv also increases. Furthermore, as is shown in FIG. 9, varying the gate-source voltage Vgs causes the mutual conductance gm to vary. Accordingly, the voltage gain Gv of the distributed amplifier can be varied by varying the gate-source voltage Vgs.
As was described above, variation in the waveforms becomes more severe in the order FIG. 8Bxe2x86x92FIG. 8Cxe2x86x92FIG. 8D. If this is applied to the operating points in FIG. 9, then the variation in the waveforms becomes more severe in the order bxe2x86x92cxe2x86x92d. It is seen from this that the variation in the waveforms becomes more severe as the operating point is positioned in a region where the slope of the gm curve is larger. In a case where the amplitude of the input signal IN is 0.5 Vpp as in the example shown in FIG. 8, the gate-source voltages Vgs of the source-grounded transistors 701-1 through 701-8 swing about the operating point in the range of xc2x10.25 Vpp. Accordingly, if the curve is inclined in the vicinity of the operating point, the mutual conductance gm fluctuates as the gate-source voltage Vgs swings; as a result, the voltage gain also fluctuates. For example, in the case of the operating pint d and operating point c in FIG. 9, the voltage gain Gv is reduced in the vicinity of the low level of the input signal IN, and the voltage gain is increased in the vicinity of the high level of the input signal IN. Here, in the distributed amplifier shown in FIG. 7, the output signal OUT is inverted with respect to the input signal IN. Accordingly, the output signal OUT has a small voltage gain Gv in the vicinity of the high level, and the voltage gain Gv is large in the vicinity of the low level of the input signal IN. Accordingly, in the case of the operating points d and c, an output signal OUT with a waveform in which the vicinity of the low level is emphasized is generated. As a result, it appears that distortion of the output signal OUT with respect to the input signal IN is generated, so that the cross point is shifted upward.
Meanwhile, in the case of the operating point b, the gm curve is more or less left-right symmetrical in the region of xc2x10.25 Vpp. Accordingly, the voltage gain Gv in the vicinity of the low level of the input signal IN and the voltage gain Gv in the vicinity of the high level of the input signal IN are more or less the same, so that there is almost no deviation of the cross point. Accordingly, a good output waveform can be obtained.
Depending on the application, there may be cases in which the position of the cross point is deliberately set as a position that is shifted from the vicinity of the center between the high level and the low level. The cross point is shifted from the set position in accordance with variations in the potential TMI. Accordingly, the output signal waveform becomes a waveform that differs from the intended waveform.
In the distributed amplifier shown in FIG. 7, as was described above, the voltage gain Gv can be controlled using the potential TMI; however, the variation in the waveform of the output signal OUT may become severe depending on the set value of the voltage gain Gv. This is an extremely serious problem in applications in which a good waveform is required.
It is an object of the present invention to provide a distributed amplifier in which there is little deterioration in the output waveform accompanying control of the voltage gain.
Accordingly, the distributed amplifier of the present invention comprises a plurality of amplifying circuits which input signal supplied from a common input terminal, and which supply the signals to a common output terminal following amplification, wherein the amplifier is constructed so that the gain values of the amplifying circuits are set separately for each amplifying circuit.
In the distributed amplifier of the present invention, the gain can be set separately for each amplifying circuit. The gain of the distributed amplifier is the composition value of the gains of all amplifying circuits. The distributed amplifier of the present invention can obtain a output signal of good waveform when each gain of amplifying circuit is set to a value which causes substantial no distortion of waveform.